1. Field of the Invention
The present invention relates to ball mills for the milling of flowable materials, and more specially to such a mill of the type having a centered milling space between a stator and a rotor through which flows freely moving grinding media and the material to be milled, that is supplied through an inlet and is let out through an outlet at which there is a separator for separating and keeping back the grinding media from the milled material so that the media may be run through a return duct to the inlet.
2. Description of the Prior Art
The German Offenlegungsschrift specification No. 2,811,899, which corresponds to U.S. Pat. No. 4,304,362, describes a ball mill designed on these lines and in the case of which the material to be milled and the grinding media are kept circulating round a displacing ring of wedge-like cross section, the material outlet and the material inlet being placed quite near to the axis of the mill so that the grinding balls separated by the separating unit may be run back through a return duct and then into the material in the process of being milled. In this case, the return duct is designed running through a disk on the rotor and is radial so that the media are moved therethrough by centrifugal force.
In order, in the case of mills of this type, to make certain of a more or less even distribution of the material to be milled and of the grinding media, the rate of flow of the media and of the material have to have a certain relation to each other. The rate of flow of the material being milled may for example be changed as desired by changing the supply pressure and the speed of the rotor. Although the grinding media are moved by the material to be milled, the driving force acting in this respect is more importantly controlled by the viscosity of the material. Furthermore, the speed of the rotor has a limited effect on the circumferential speed of the said material and of the media. Because, however, the relation between the circumferential speeds cannot be controlled to keep it at a constant value, the operation of the mill, that is to say its effect on the material supplied thereto, will not be constant. Although some attempts have been made to make changes in the speed of circulation of the grinding balls by changing the size of the return duct or ducts, such adjustments are as a rule only possible when the mill is empty. Furthermore the adjustments are not great enough.